test/py: use default load address for tftp

On x86_64 the size of the file u-boot loaded by the tftp test has grown in
size such that when loading the file to 0x200000 it overwrites a memory
area reserved for PCI.

If no load address is specified for tftp do not use the ram base address
(or if zero 0x200000) but the default address.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>